How much faster could I got with a higher pitch prop?

[barrbrad]

Hi I just recently bought my very first boat, and while I don't have tons of knowledge I am hopefully going to gain some on the way. My question for you guys is how much faster could I go if I purchased a 21 or 22 pitch prop? I currently have a 1994 Larson Delta-Conic with a Volvo-Penta motor 120 HP and (what I believe to be) a 17 pitch prop now.

I use the boat for cruising around on the lake as well as tubing/water sports in general is the main use. Right now towing 2 adults with 2 adults in the boat I can get it up to 30/35 mph, but if I go up to a 21 or 22 pitch will I notice a noticeable difference?

Thanks!

 

[StingRay_90V4]

I dont know anything at all about the Volvo Penta engines. But I do know that as you go up in pitch. You will drop rpms as well as loose your hole shot. If you do any amount of skiing or tubing. I would think you may want to keep a decent hole shot. If you have a 17p prop right now. Maybe a 19p would still be ok for tubing and you may gain a few extra mph. The only real way to know is to try different props. Keep in mind that it is best if you know what rpms your boat is currently running at wot. And to prop it in the engines desired range. The more experienced people with the volvos I'm sure will tell you your best rpm range to be in.

 

[Scott Danforth]

welcome aboard

please verify the motor
please verify the drive
what RPM are you currently running at WOT?
is the speed with GPS? because the speedo is a mere suggestion

most likely your propped at the top end now, and increasing pitch would actually lug the motor, dropping your top end and killing the motor.

 

[fhhuber]

insufficient information.

What RPM are you getting now?
If you are not able to get to max rated RPM of the engine you may actually slow down by adding pitch.

Caculated pitch speed vs actual GPS speed tells how well you are doing at hitting top boat speed for the prop.
If you are significantly below the mph calculated by Pitch (inches) X rpm/1000 = mph then you have excess slippage and you are going to go slower with more pitch.
(this calc assumes a small slippage factor, you should be able to hit this speed on GPS)
Note: this is RPM AT THE PROP which may or may not match engine RPM... look up your gear ratio.

its not unknown to reduce pitch, increase blade area (blade dia or number of blades) and go faster due to significant reduction of slippage.

 

[tpenfield]

Folks who play around with changing props tend to waste money and time. Without engine specifics, it is tough to say, but going from a 17 to a 21 (or 22) pitch prop is a significant change and will most likely be a bad decision. Chances are that the boat has the proper size prop on it already and dramatically changing that will just mess things up.

 

[Sea Rider]

Right now towing 2 adults with 2 adults in the boat I can get it up to 30/35 mph, at which max wot rpm ? If at max wot rpm range a higher pitch prop will lessen wot revs towards middle range, if at middle range could lessen wot rpm towards the min lugging side of engine. That's why current achieved wot rpm as loaded is important to know.

Happy Boating

 

[jimmbo]

Hi I just recently bought my very first boat, and while I don't have tons of knowledge I am hopefully going to gain some on the way. My question for you guys is how much faster could I go if I purchased a 21 or 22 pitch prop? I currently have a 1994 Larson Delta-Conic with a Volvo-Penta motor 120 HP and (what I believe to be) a 17 pitch prop now.

I use the boat for cruising around on the lake as well as tubing/water sports in general is the main use. Right now towing 2 adults with 2 adults in the boat I can get it up to 30/35 mph, but if I go up to a 21 or 22 pitch will I notice a noticeable difference?

Thanks!

You will notice a difference. A long time getting up on plane, if it does manage to plane, and a lower top speed

 

[preventec47]

In at least one specific set of conditions you could get a desirable increase in performance with a higher pitch prop. That does not mean that it would be bad for other conditions but if you are going for a scenario where you are willing to unload ballast and useless weight in the boat which will reduce the drag on the water, a higher pitch prop could gain you some noticeable extra top end speed when
you want to speed around the lake solo ... more like a race boat than a pleasure or utility etc
ski boat. It is possible and we cannot know till you try that with several people in the boat
a higher pitch prop might be the same or a little bit slower than your current prop. Another benefit
( or a higher pitch prop) when your boat is in the "low drag" configuration is that you can plane out at slightly lower rpms and running at that speed would provide for you your maximum possible range. You know like if you were leaving New Orleans and heading for Cancun etc.

 

[fhhuber]

You can randomly swap props and eventually find the best prop (which might be what you have now)

Or you can intelligently evaluate RPM vs speed.

First thing you need to answer:

Does speed noticeably decrease while towing vs with everyone on the boat? If this happens, then going up in pitch is exactly the wrong thing to do.

Towing the tubers = more drag and if you have the prop for max speed when not towing , you will not go faster while towing buy increasing pitch. All you will do is overheat the engine and slow down.

You MIGHT have higher speed when towing by reducing pitch and increasing blade area, sacrificing some top speed when not towing, for better speed while towing and ability to get up on plane (slightly) faster.
Again, you could randomly play with different props, and find out that you already have the best prop for towing...